The invention relates to the construction of an internal combustion engine for a marine propulsion system. More specifically, the invention relates to a reciprocating force balancer which is connected to the crankshaft of the marine engine and is useful in counteracting the shaking force created by a pair of engine pistons.
In a typical in-line, twin cylinder two-stroke engine, the pair of pistons are connected to a crankshaft 180.degree. apart, such that as the first piston moves upward in its cylinder, the second piston is moving downward in its cylinder. With this type of arrangement, the inertial force created by the movement of the first reciprocating piston is canceled by the second piston that is moving in the opposite direction.
The above described 180.degree. crankshaft creates uneven firing order in a twin cylinder four-stroke engine, since in a four-stroke engine, each piston has a power stroke only once for every two revolutions of the crankshaft. In a four-stroke engine with a 180.degree. crankshaft, after the first piston fires, the second piston will fire after the crankshaft rotates 180.degree.. After the second piston fires, the first piston will not fire again until the crankshaft has rotated another 540.degree.. Thus, in a four-stroke engine with 180.degree. crankshaft, the pair of pistons do not fire in an even pattern, which causes the engine to run in an undesirable manner.
In order to avoid uneven firing order in a twin cylinder four-stroke engine, there is a need to have a 360.degree. crankshaft in which both the pistons move up and down in unison. In a four-stroke engine with a 360.degree. crankshaft, after the first piston fires, the crankshaft will rotate 360.degree. before the second piston fires, since the pair of pistons move up and down together. Once the second piston has fired, the crankshaft will rotate another 360.degree. before the first piston fires again. Therefore, when a 360.degree. crankshaft is used in a twin cylinder four-stroke engine, one of the pistons fires for each revolution of the crankshaft.
When a pair of pistons are connected to a 360.degree. crankshaft to move up and down in unison, the inertial forces generated by the reciprocating movement of the combined mass of the pair of pistons creates an inertial shaking force which causes the engine to vibrate. The large magnitude of the shaking force is often difficult to isolate in the mount system of the engine. In the case of an outboard motor, the shaking force causes vibration in the tiller handle and/or the boat, making operation unpleasant.
Several methods have been developed to attempt to counteract the shaking forces created by the pair of reciprocating pistons. The first method of eliminating the shaking force is to change the firing order of the pistons such that the pistons move up and down at opposite times with respect to one another. However, as previously discussed, when the piston timing is modified as such, after the first cylinder fires there is a delay until the crankshaft rotates 540.degree. before the next cylinder fires. Configuring a twin cylinder, four-stroke engine in this manner results in uneven firing, which is not a viable solution.
A second method of reducing or eliminating the shaking force is to construct an engine in which the pair of pistons are opposed to each other. When a pair of pistons are opposed to each other, they move in opposite directions along a single line of action. If the pistons are opposed, one of the pistons will fire for each revolution of the crankshaft. When the pair of pistons are opposed, the inertial force created by the movement of the first piston tends to counteract the inertial force created by the movement of the second piston. However, in an opposed engine, each of the cylinders extends in an opposite direction from the crankshaft, greatly increasing engine length, which typically is an undesirable result. Additionally, in an opposed engine having a single carburetor, the manifolds for the intake and exhaust are complicated compared to an in-line configuration.
A third method of reducing or eliminating the shaking force is to use a force balancer mechanism to counteract the inertial force created by the pair of pistons. Suzuki et al U.S. Pat. No. 4,846,124 discloses a balancer mechanism having a driver gear mounted to the crankshaft and operatively coupled to a pair of balancer shafts. The first and second balancer shafts rotate at the same speed as the crankshaft to counteract a portion of the inertial forces produced by the reciprocating mass of the pistons. The balancer mechanism disclosed in the U.S. Pat. No. 4,846,124 incorporates a complex series of gears and belts which combine to rotate the pair of balancing shafts. This complex arrangement requires many additional components and is therefore cost prohibitive. The complex connections between the crankshaft and the balancing shafts also require very precise manufacturing tolerances and careful gear design to avoid noisy operation. Finally, since the balance gear/shaft mechanism rotates at the engine speed, the balance mechanism does nothing to eliminate second order and higher shaking forces.
Therefore, it can be appreciated that a force balancer that can be incorporated into a marine engine, particularly a two-cylinder four-stroke engine, to balance the shaking forces created by the reciprocating pistons would be desirable. Particularly, a force balancer which consists of few parts and can be easily constructed and positioned in a marine engine would be particularly desirable.